The protein ricin has been shown to be an effective anti-tumor agent. It is a heterodimer consisting of a lectin B chain which binds to cell surface receptor sites and causes the toxic protein to be taken into the cell endocytosis. The A chain kills the cell by inhibiting protein synthesis, apparently by an enzymatic disruption of the ribosomal elongation factor binding sites. This chain has been combined with hormonal receptor proteins to kill specific cell lines by chemical micro-surgery, and has been linked to specific antibodies to form a specific anti-carcinoma agent. We have produced a 4 Angstroms electron density map of ricin and location two carbohydrate binding sites on the B chain. Analysis of this structure led us to realize that ricin B chain is a gene duplication product. We have also obtained a 2.8 Angstroms electron density map and propose to continue our efforts at producing a molecular structure from it. The structure will be analyzed and compared with the structure of the anti-viral protein PAP presently under study in our laboratory and with the structure of the toxin modeccin (which we have crystallized and propose to solve by x-ray diffraction). These comparisons, together with kinetic and enzymological studies, should produce a molecular mechanism of action for this class of toxic enzymes and should shed light on how the heterodimer protein recognizes cell surfaces. This information in turn should provide an explanation for the protein's observed action and may suggest steps to be used in rational drug design.